Teardown: Leviton Gound fault circuit interrupter Page 8
How advanced lGBT gate drivers simplify high-voltage, high-current inverters Page 18
FEBRUARY 2018
Power Electronics Handbook
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LDO Regulators with Low Quiescent Current
Allow Your Application to Remain in Standby/Sleep Mode Longer
Standby/Sleep Mode
In a world that runs on battery power, the ability to drastically lower power consumption during standby modes or sleep mode is no longer a luxury. This is especially true in mobile medical applications that have extended use between charging cycles. Consumers are also looking to squeeze every bit of battery power they can from each charge cycle, and that demand requires efficient low dropout regulators to optimize standby power usage. Microchip has a large selection of Low Dropout (LDO) regulators with the highest performance for quiescent current. We now offer the lowest quiescent current LDO in the market, consuming only 20 nA (typical) while operating with no load condition. By reducing the quiescent current, these LDOs enable your application to stay in standby/sleep mode up to 20 times* longer than competitive devices. These LDOs are available in small packages, ranging from 1 × 1 mm DFN to SOT23 packages, with capacitor-less operation** to minimize the footprint and reduce the PCB usage for congested real estate. These LDOs are ideal for batterypowered applications such as wearables, key fobs, GPS, handheld equipment, hearing aids, Bluetooth® earphones and more.
www.microchip.com/low-iq-ldo *compared to 1 uA quiescent current LDOs **MCP1711 and MIC5231 only The Microchip name and logo and the Microchip logo are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. All other trademarks are the property of their registered owners. © 2018 Microchip Technology Inc. All rights reserved. 1/18 DS20005940A
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POWER ELECTRONICS HANDBOOK
Hiring hint: Look for soft skills in young engineers Do
schools teach young engineers the right skills? It is easy to find a lot of opinions on that subject from academics. It is more difficult to find assessments from experienced engineers who actually work with new grads. That’s why the comments of Richard Lukaszewski are worth hearing. As a holder of over 30 patents and an MSEE, he’s hired quite a few young engineers at Rockwell Automation where he is a manager of new product development for power electronics. “Young engineers generally seem OK on the harder engineering skills. It is the soft skills where they are lacking,” he says. In particular, he wishes young engineers had better writing and reasoning skills. “They’ll often write reports that are either too detailed or lacking enough particulars. They also tend to present a whole lot of data but no conclusions or recommendations they’ll stand behind. That’s not why I’m paying them.” Lukaszewski says he puts more emphasis on soft skills because the character of design engineering has changed dramatically in just the last decade. “It has become much more process-oriented. The design part is probably a small portion of the day,” he says. “But the vast majority of a project consists of testing and communicating updates. So 80% of your time is in working with the other cross-functional teams to make sure the design is moving along.” The changes in the nature of product development have led Lukaszewski to look for qualities in new hires that differ from those of a few decades ago. “One of the things I look for in a young engineer is the ability to talk and negotiate things. Sometimes when you read a spec it is kind of cold. You need to get a better sense of it by talking with people so you understand the tradeoffs. That demands an ability to negotiate to the spec with the product management people,” he says. “For example, we have motor drives with circuit boards that must carry 150 A. That’s high for a circuit board. It takes
a lot of interaction between the power engineer and the board layout personnel to get something that’s right.” And there are some aspects of design engineering that are tough to learn in school. “My highest performers know what ‘done’ looks like. They don’t gold-plate a design. Other engineers will work on a design to a point where it is beyond what is asked for,” says Lukaszewski. “Some people just seem to have a natural ability to do this.” But some soft skills can indeed be learned. “Engineers have to go through one full design cycle to understand what it is all about, get the cadence down, and understand what is expected of them,” Lukaszewski says. To that end, he thinks a change in the engineering education process might be in order. “The people we interview with a BS degree usually have done a design project in their senior year. I would like to see those projects more integrated with other disciplines so there is interaction with business, mechanical engineering, and industrial design people. That would take the project out of the realm of, ‘I built this circuit,’ and into the area of real development,” he says. “If nothing else, it would teach that you have to write requirements before you design anything.”
LEE TESCHLER, EXECUTIVE EDITOR
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INSIDE EEWORLDONLINE.COM
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THE POWER ELECTRONICS HANDBOOK FEBRUARY 2018
23
8
02 HIRING HINT: LOOK FOR SOFT SKILLS IN YOUNG ENGINEERS
33 23 ELECTROLYTIC
FAULT INTERRUPTER
OUTPERFORMS GaN Silicon carbide and gallium nitride transistors both have their niche, but it pays to understand the applications in which each excels.
DRIVERS SIMPLIFY HIGH-
6
IGBTs have nuanced operations associated with high current densities and temperature that force IGBT gate drivers to incorporate sophisticated features.
DESIGN WORLD — EE NETWORK
Contents Power Electronics HB 2-18 V1.indd 6
Sometimes the best way of handling specialized power needs is through custom equipment optimized for the situation at hand.
WHY WORKING VOLTAGE MATTERS IN SAFETY APPROVALS
COOLING FANS
The life of most fans is set by the shaft bearings. Simple comparisons show why different bearing technologies can exhibit widely varying MTBF figures.
33 DESIGNING WITH BRICKS BASE-PLATE-COOLED CONVERTER MODULES
Power converter modules can simplify equipment design, but engineers may still need to look at EMC emissions and inputsurge filtering.
2 • 2018
Here are a few tips for selecting resistors that will accurately gauge current.
39 TOUGH TERMINOLOGY:
30 BEARING TRADE-OFFS FOR
INVERTERS
WITH SHUNT RESISTORS
IN POWER SUPPLY DESIGN
18 HOW ADVANCED IGBT GATE VOLTAGE, HIGH-CURRENT
The traditional cylindrical packages that have characterized electrolytic caps are giving way to flat packs optimized for small spaces.
26 FIVE CHALLENGES
14 WHERE SiC
36 MEASURING CURRENT
CAPACITORS GO SVELTE
08 TEARDOWN: LEVITON GROUND GFCIs detect hazardous current paths to ground and ground-to-neutral faults. They are relatively simple but have circuit details that can be tricky to figure out.
43
Organizations that test power circuits for safety have specific definitions of working voltage and other parameters that bear heavily on how products get safety certifications permitting their sale.
43 HIGH-TEMPERATURE, HARSHENVIRONMENT TANTALUM CAPACITOR TECHNOLOGIES
Solid-tantalum capacitors have evolved with better design, construction, and testing methods to handle emerging requirements for harsh, hightemperature applications that put a premium on reliability. COVER PHOTOGRAPHY BY ALLISON WASHKO
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POWER ELECTRONICS HANDBOOK
Teardown: Leviton Ground fault circuit interrupter GFCIs detect hazardous current paths to ground and ground-to-
LEE TESCHLER, EXECUTIVE EDITOR
neutral faults. They are relatively simple but have circuit details that can be tricky to figure out.
Bathrooms,
kitchens, and other rooms with access to water are required to employ ac outlets with ground-fault circuit protection. GFCIs disconnect the output if they sense a hazardous current flow. The GFCI function generally is controlled by a single IC. The mechanical make-up of most GFCI brands is quite similar. The usual approach is to put the electrical connections to hot and ground wires through a set of electromechanical switch contacts. The contacts feed ac to the outlet in normal operation. When the GFCI detects a problem, it powers a solenoid which disconnects the contacts. Pushing a manual reset button reestablishes the connection to the outlet.
Third-wire ground connection
DECONSTRUCTION A majority of GFCI outlets are held together with four screws through the back plastic housing. Removing them allows the front plastic face of the device to lift off, revealing the metal
Hole for GFCI test lever
Four screws hold the GFCI together. Removing them allows the outlet face to come off. Visible here is the metal attachment frame work that holds the connection points for the third-wire grounding and ground lugs. The test button on the outlet face pushes through the hole in the framework to connect a resistor that actuates a
Ground lug
solenoid and disconnects the outlet contacts.
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KNOWLEDGE IS POWER
Massive power density in the smallest packages
Microchip Technology now offers an integrated switching power module designed specifically for height-constrained telecom, industrial and solid-state drive (SSD) applications. These products come in an impressive thermally-enhanced package that incorporates inductors and passive components into a single, molded power converter. The slim packages simplifies board design, saves space and eliminates concern over passive components that may introduce unexpected electromagnetic interference (EMI). Highlights •
Variety of module package offerings (small to large, fit to application)
•
High power density with integrated magnetic and passive components
•
Performance (efficiency, thermal, transient response)
•
Reliable (power and thermal stress tested)
•
Low EMI (CISPR 22 Class B ratings on modules)
www.microchip.com/powerpromo The Microchip name and logo and the Microchip logo are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. All other trademarks are the property of their registered owners. © 2016 Microchip Technology Inc. All rights reserved. 9/16 DS20005637A
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POWER ELECTRONICS HANDBOOK
A better look at the grounding connections and reset button spring.
Fastening framework and ground connection
Reset button spring Reset button actuator
contact assembly and attachment backbone. The plastic reset switch lifts off to reveal a spring behind it and the mechanism that pushes the electromechanical switches back into position after an actuation. The metal frame containing the attachment points and the third-wire ground connections then lifts off the plastic housing. The connection points for the hot and neutral plug connections lift out as well. Then the front half of the plastic housing can be lifted off to reveal a view of the electromechanical contacts and the attachment points for the incoming hot and neutral wires. The electrical components sit on a single circuit board that just lifts out of the plastic housing. A circuit board forms the mechanical backbone for the rest of the GFCI components, including the sensing toroids, wire connection points, electromechanical relay, and circuit components. It is a single-sided board with one main chip and several discrete components. The chip on this particular GFCI is a groundfault interrupter control device from Fairchild Semiconductor (recently bought by ON Semiconductor). The other main device on the board is a silicon-controlled rectifier (SCR) from NXP used to trigger the solenoid to disengage electrical contacts in the event of a fault. The explanation of how the GFCI operates is basically a description of the Fairchild chip functions. The chip contains a precision op amp whose inputs connect to a wound toroid. Hot and neutral lines pass through the center of the toroid. The operative principle is that the current through the hot and neutral lines should be identical. If they are, there is no net magnetic field generated, and there should be no current induced in the toroid windings. But if the two currents are not the same, there’s a problem. The assumption is that some of the current could be flowing through a human operator.
Behind the reset button is a spring and an actuator that pushes the electromechanical relay to reset it.
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TEARDOWN
FAIRCHILD GFCI CHIP BLOCK DIAGRAM AND PIN-OUT
Among the features visible in the GFCI chip block diagram are the four op amps that actuate the SCR on the detection of a fault current and the diode bridge for internal chip power.
The current generated in the toroid windings goes to the precision op amp which converts it to a voltage. Two comparators In UL943, UL specifies this resistance network on the chip are connected to the sense amp output. They are for testing GFCI reactions to fault currents. configured as a window detector. When the current from the sense Fairchild made use of these definitions by toroid exceeds a preset level, the window detector output starts a realizing that they could constitute a positive delay circuit. If the sense current exceeds the trip current for a time feedback loop when current flows through the longer than the preset delay interval, the chip triggers the SCR. The ground connections. SCR is connected to the solenoid which actuates the electromechanical contacts, disconnecting the outlets UL TEST CIRCUIT SHOWING GROUNDfrom the line and neutral wires. TO-NEUTRAL FAULT RESISTANCE There is a second toroid used as a current sensor in the GFCI. Judging by comments we’ve seen on forums, there is some confusion about its purpose and how it works. The point of the second toroid is to sense current flowing from ground to neutral, rather than a difference in the line and neutral current. One point of confusion among some online commenters is how current can flow in the ground lead without causing a difference in the hot/neutral current that the first toroid would sense. The answer: One can envision a scenario in which current flows from hot to load, then to neutral, and then to ground. There would potentially be no difference in the line and neutral current. So the GFCI circuit must sense this fault current with the operating assumption that the hot and neutral currents are the same.
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POWER ELECTRONICS HANDBOOK GFCI IN THE PRESENCE OF GROUND-TO-NEUTRAL FAULT
The Fairchild reference circuit for the GFCI controller chip with the positive feedback loop called out, present when current flows through the ground connection.
Ground current sensor
Hot & neutral connections thru sense coils
MOV Ground fault sensor
Solenoid
GFCI makers can use several methods to detect ground to neutral current. The Fairchild chip uses a scheme which makes use of the definition of fault current resistance as spelled out in UL 943, the U.S. standard for ground-fault circuit interrupters. The standard specifies that GFCIs trip with fault currents of 6 mA for specified ground fault resistances and for a specified combination of ground to neutral resistance and wire resistance between load/neutral and earth ground. In UL tests, the resistance of the combination of grounded and grounding conductors of the cable or cord are quite low, 0.4 and 1.6 Ω at most.
Visible in this view of the GFCI assembly and its plastic housing are the two wire-wound toroid sensors which mount concentrically. The outer toroid appears to have fewer windings so it is probably the ground current sensor. The two metal straps make up the arms of the electromechanical relay and extend down through the toroids, terminating on the circuit board below. Also visible is the solenoid for the relay.
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TEARDOWN
The single-sided circuit board holding the GFCI electronics. The big solder piles at the bottom are the termination of the relay arms after they pass through the
NXP SCR
current-sensing toroids.
Fairchild GFCI chip
The conductors for the outlets just sit in the plastic housing. They just lift out. Visible on them are the contact points for the
Solder connections from conductors thru sense coils to relay arms
In the Fairchild GFCI reference circuit, the fault current through the ground connection creates positive feedback that causes the sense amplifier to oscillate. Oscillations for a period longer than the time delay window cause the SCR to trigger, actuate the solenoid, and disconnect the outlets from the input wires. The oscillations result from a tank circuit formed by the toroid secondary inductance and an external capacitor. In the Fairchild reference design, the oscillation frequency is 8 kHz. There are a few other resistors and capacitors on the GFCI board and a protective metal-oxide varistor on the input ac line. The R’s and C’s determine factors such as the amount of fault current that will trip the device, the amount of time the fault current must exist to trip the device, and the maximum current through the GFCI controller chip. A final point to note is that the Fairchild chip is powered only during the positive half period of the line voltage, but it can sense current faults independent of its phase relative to the line voltage. Similarly, the gate of the SCR is driven only during the positive half cycle of the line voltage.
electromechanical relay.
Outlet conductors
Relay contacts
Front of housing
Relay contacts
REFERENCES Fairchild GFCI data sheet www.fairchildsemi.com/datasheets/RV/RV4141A.pdf NXP SCR www. pdf.datasheetcatalog.com/datasheet/NXP_Semiconductors/BT168GWF.pdf
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Wire connection points A view of the GFCI assembly with the front cover removed. There is a good view of the relay contacts through which the outlet connects to the incoming hot and neutral wires.
2 • 2018
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POWER ELECTRONICS HANDBOOK
Where SiC outperforms GaN ZHONGDA LI, UNITED SILICON CARBIDE INC.
Silicon carbide and gallium nitride transistors both have their niche, but it pays to understand the applications in which each excels.
SiC ‘TRENCH’ CELL
GaN ‘LATERAL’ CELL
The typical construction of a SiC and GaN JFET cell.
Wide
band-gap (WBG) devices such as silicon carbide (SiC) and gallium nitride (GaN) are the hot topics of the moment, promising anything from universal wireless charging to power converters shrunk to almost no size. However, these are projections from theoretical performance that don’t always correspond with reality. Let’s take a step back and just outline what WBG devices are. Semiconductors have bound electrons which occupy distinct levels around an atomic nucleus – valence and conduction bands. Electrons can move up to the conduction band and be available for current flow, but they require energy to do so. In WBG devices this energy requirement is much greater than with silicon. For example, SiC requires 3.2 electron-volts (eV) compared with silicon (Si) at 1.1 eV. The additional energy required to move WBG device electrons into the conduction band brings higher voltage breakdown performance compared with silicon of the same dimensional scale.
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2 • 2018
For the same reason, SiC can withstand higher temperatures (thermal energy) before failure and also, as a material, has a thermal conductivity about 3.5 times better than Si. In practice these attributes promise high temperature operation at high voltage and power levels. Devices initially available in SiC were simple diodes, but the material technology has advanced to allow fabrication of JFETs, MOSFETs and even bipolar transistors. Special geometric configurations used in the construction of SiC devices have brought additional advantages. For example, a SiC JFET with a vertical trench construction exhibits low ON-resistance compared with a GaN HEMT cell with lateral construction. Although the devices are normally “ON” with zero gate voltage, a cascode arrangement of a co-packaged Si MOSFET with the SiC JFET gives a hybrid device with gate drive voltages compatible with Si MOSFETs that retains the advantages of WBG devices. (As a quick review, a cascode arrangement consists of a common-emitter stage
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WHERE SiC OUTPERFORMS GaN
feeding a common-base stage. The heavily loaded common-emitter stage has a gain of only one, overcoming the Miller effect. A cascode amp has a high gain, moderately high input impedance, and high output impedance, and a high bandwidth.) Coming later than SiC, GaN has had slow adoption because of cost, yield, and reliability concerns. It is certainly theoretically capable of switching at higher speeds than SiC or Si, with its much higher electron mobility. But GaN thermal conductivity is lower than that of Si, so its power density potential is limited. Currently SiC devices are commonly used at around 650 V through 1.2 kV and higher, while GaN is limited to around 650 V where it struggles to compete with less expensive, more robust, and more mature SiC offerings at the same voltage. GaN suppliers are targeting lower voltage/ power markets that include data centers, EV/ HEVs, and photovoltaics. However SiC also addresses these market areas, especially in applications for bi-directional dc-dc converters and totem pole PFCs. (As another quick review, a totem pole circuit uses one transistor to drive the output high, another connected below it to pull the output low. In totem pole PFCs, totem poles replace the four rectifier diodes to, among other things, eliminate the diode voltage drop.) SiC addresses the IGBT market at high powers and voltages, and GaN targets the
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POWER ELECTRONICS HANDBOOK POWER TECHNOLOGIES AND WHERE THEY FIT
The market analysis firm IHS thinks the uses for power devices might split along the lines of power and operating frequencies, depending on the economics of future device cost reductions.
lower-power but higher-volume and cost-sensitive markets of Si MOSFETs. These applications all demand WBG benefits – potential higher efficiencies and smaller size, but inexpensive and reliable, and ideally second-sourced. SiC is well established in the supply chain. SiC parts are available even from high-service distributors, whereas GaN parts have yet to become mainstream. The market analysis firm IHS Markit predicts this relative split in usage will remain into the mid-2020s, with the combined WBG market reaching $3.5B, of which GaN will represent only about $500M. Even if GaN voltage ratings improve, SiC has an edge in industrial systems because it can withstand voltage avalanche conditions, as can happen with inductive loads. Manufacturers have extensive data showing SiC reliability in the prescence
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of voltage overstress. Makers of GaN devices make no claims except to say maximum voltage should not be exceeded. A more tangible difference between SiC and GaN is the packaging; SiC parts are commonly available in TO247 and T0-220 styles. Consequently, they can drop in as replacements for Si MOSFETs in existing designs. However, GaN device manufacturers recognize that plastic packages, with their inherent speed-limiting lead inductances, can be a barrier for performance. They have therefore mostly opted for surface-mount, singlesource, chip-scale packaging, which limits their adoption in new designs. Here, the system design can take advantage of GaN device properties to make use of smaller passive components, particularly magnetics and capacitors.
2 • 2018
Ironically, the real constraints imposed by meeting EMI standards and keeping dV/dt levels manageable often force designers to slow switching speeds with gate resistors. For example, dV/dt levels of 100 V/nsec are easily possible with SiC and GaN. But from I = C dV/dt, this dV/dt produces a 10-A current spike into just 100 pF of stray capacitance. Similarly, high di/dt values produce voltage spikes across connection inductance. THE STATE OF THE ART SiC devices are commonly available with 650 V and 1,200-V ratings at currents up to around 85 A with on-resistances of around 30 mΩ as cascodes as well as super cascodes – series-connected JFETs with ratings exceeding 3.5kV. Single devices up to 1,700 V at around 70 A and 45 mΩ are available, but as MOSFETs rather than JFET
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WHERE SiC OUTPERFORMS GaN
SiC IN POWER FACTOR CORRECTION cascodes. This means their internal body diode is relatively slow, unlike cascodes, and often must be bypassed with a costly fast external diode when the application requires it, as for example in bridge circuits. GaN devices top out at 650 V with about a 60 A and 25 mΩ rating, equivalent to many SiC parts but theoretically capable of faster switching. Interestingly, today’s GaN devices with 100-V ratings are no better than traditional MOSFETs for on-resistance. So their speed advantage must be enough make it worth using GaN rather than commodity MOSFETs at this level. IHS data predicts a significant rise in WBG device design-ins, although IGBT and traditional MOSFET sales are expected to also increase in a growing market. But there is a debate about how different WBG devices might dominate particular applications. The high temperature capability of WBG devices with potentially fast switching and low losses makes them good candidates for military and industrial applications where performance is key. Highenergy bridge circuits are obvious applications in inverters, welding equipment, class D audio amplifiers, motor drives, and more. One application in particular that sees major benefits is the bridgeless totem-pole PFC circuit. Here, versions using Si technology have been limited by the slow performance of body diodes in the MOSFETs. This slow response forces operation in a critical conduction mode, which in turn produces high peak currents and high EMI. But substituting cascode SiC JFETs allows operation in continuousconduction mode. The result is better efficiency, smaller inductors, and fewer EMI problems. One such circuit at 1.5 kW, operating from a 230 Vac line, showed an impressive efficiency of 99.4%. In high-power applications, operation in the presence of transient short-circuits and overvoltages is a major concern. A typical cascode SiC JFET has excellent qualities in this respect. High currents cause a pinch-off effect which limits current to a saturation level. Additionally, heating produced by the current reduces the channel conductivity, giving rise to self-limiting. The high allowed junction temperature also helps here. For overvoltages, the SiC JFET gate-drain diode conducts, causing current flow in the gate drive circuit and turning the JFET channel on to clamp the overvoltage. Again, the inherent high temperature rating of the SiC die gives a good margin of safety for significant avalanche energy levels even in the relatively small die sizes encountered. Manufacturers such as United Silicon Carbide Inc. have proven their SiC offerings are robust with parts qualified to 1,000 hours of operation biased into avalanche at 150°C. As an additional confidence measure, all parts go through 100% avalanche at final test.
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SiC devices in a bridgeless totempole PFC stage. Here diodes D1 and D2 are part of the PFC circuit but don’t handle ac rectification. The JFETs and MOSFETs function as the rectifying elements, replacing the traditional diode bridge.
REFERENCES IHS Markit SiC and GaN power semiconductor report www. technology.ihs.com/521146/sic-gan-power-semiconductors-2016 Application note, 1.5 kW Totem-pole PFC Using 650V USCi SiC Cascodes unitedsic.com/wp-content/uploads/2016/11/TotemPole-PFC-AppNotes.pdf Robustness of SiC JFETs and Cascodes unitedsic.com/wp-content/uploads/2016/02/ bp_2015_05-Robustness-of-SiC-JFETs.pdf
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POWER ELECTRONICS HANDBOOK
How advanced IGBT gate drivers simplify high-voltage, high-current inverters
TOM ZEMITES, KIM GAUEN, NXP
IGBTs have nuanced operations associated with high current densities and temperature that force IGBT gate drivers to incorporate sophisticated features.
The
automotive industry is to minimize system cost, minimize PCB lGBT EQUIVALENT moving inexorably towards area, ensure the system meets ISO 26262 CIRCUIT electrification. As automakers move requirements, and design flexibility for from electric assistance to full re-use. autonomy, the trend in EV invertors is toward higher system efficiency and MINIMIZING SYSTEM COST functional safety using technology that It may be useful to review the major also lowers costs. differences between IGBTs and MOSFETs A key consideration in the EV because some of these differences dictate inverter market is the need for smaller the functions a GDIC must provide. First, and less expensive IGBT modules IGBTs and MOSFETs are both controlled to help lower costs. Consequently, via an isolated gate and both behave the industry is moving to boost IGBT similarly. However, IGBTs use minority current density by shrinking the length carriers to conduct current whereas A typical equivalent circuit for an of the gate channel. The shorter MOSFETs are majority carrier devices. Use IGBT. Note that there is no body channel facilitates reduced IGBT die of minority carriers allows IGBTs to support diode as in MOSFET equivalent size while lowering forward voltage a high current density. circuits. drop. Smaller die size makes for However, IGBTs have a more smaller modules that cost less. Lower complicated structure than MOSFETs. forward voltage drop will reduce IGBT power dissipation by They have a stored charge that creates a turn-off tail and nearly 15%. With smaller modules, lower cost, and increased some accompanying losses. IGBT switching behavior also efficiency, we have the perfect trifecta. What could be changes with junction temperature. An IGBT is more prone wrong with this picture? to latch-up because it has a four-layer structure, and this The trade-off for IGBTs having a high current density is may also restrict the device safe operating area. IGBTs have a greater energy density when conducting overcurrents or a higher gain than do MOSFETS, resulting in more shortduring short circuits. The greater energy density can quickly circuit current for a given gate voltage. Similarly, IGBT power stress, if not destroy, an expensive IGBT module. It becomes density is greater during a short-circuit fault. Finally, ordinary necessary to quickly sense the overcurrent condition, make IGBTs lack an integrated body diode (though new “reverse the correct assessment, and softly shutdown the IGBT. conduction” IGBTs do have one). There are four key requirements for an ideal gate With these factors in mind, one can understand how driver integrated circuit (GDIC). They include being able a GDIC can help minimize EV inverter cost. In the newest
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POWER ELECTRONICS HANDBOOK
lGBT MODULE AND PARASITIC INDUCTANCES
One reason GDICs require flexibility is the widely varying inductance specifications of IGBT modules. Emitter and collector inductance greatly affect switching behavior and peak Vce at the terminals and the module die. A common emitter parasitic inductance greatly affects switching speed. Gate inductances create noise while also degrading and complicating gate-emitter monitoring and control. Additionally, inductances vary from die-to-die for paralleled d e .
generation IGBTs, the magnitude of fault current rises as the current density increases. This mandates that the newest generation IGBTs integrate ISENSE outputs enabling quick sense of an overcurrent condition. The GDIC can improve short circuit protection by monitoring the IGBT ISENSE pin
and providing quick detection (less than 2 µsec) and shutdown during an overcurrent episode. The GDIC implements two-level turn-off to reduce gate voltage during a fault detection, thereby reducing the peak fault current. (As a quick review, a short-circuit or overcurrent in the load can cause a large voltage overshoot across the IGBT at turn-off that can exceed the IGBT breakdown voltage. By reducing the gate voltage before turnoff, a two-level turnoff limits IGBT current and reduces the potential over-voltage.) Soft shutdown can be used to gently turn-off the gate voltage once the fault is validated. The GDIC can also lower the amount of heat the IGBT and the printed circuit board (PCB) radiate by boosting overall
inverter efficiency. One efficiency boosting measure is to integrate gatedriver transistors within the GDIC. This not only reduces component cost and PCB area, it also provides direct and independent control of the charging and discharging paths. This also minimizes the delay before protection kicks in and allows for a close monitoring of the gate voltage. Rail-to-rail control can be used to determine on-state voltage. Off-state voltage is pulled to the negative supply and not to a diode drop above it. A GDIC can also implement an active Miller Clamp (AMC) feature which minimizes system cost in several ways. (Again, as a quick review, turn-on or turn-off of an IGBT can cause high dVCE/dt. Displacement currents flow through the
GDIC OPERATIONS AND POWER SUPPLY ARCHITECTURE
A GD3100 GDIC as it might be configured in an EV inverter. Note the functions available for eliminating the need for a negative gate supply and gate-drive transformer.
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A D VA N C E D l G B T G AT E D R I V E R S
BLOCK DIAGRAM AND PINOUT
A block diagram of the GD3100 GDIC. Note the galvanic isolation between logic blocks one and two.
IGBT parasitic capacitances and may unintentionally turn-on the IGBT. A Miller clamp sinks the Miller current across a low impedance path in this high dv/dt situation.) A dedicated AMC pin allows designers to omit the negative gate drive supply that is otherwise often needed. Omitting the negative supply saves PCB area while eliminating a few components. More importantly it eliminates the gate drive losses (an estimated 35% extra gate power) associated with a negative supply. The result is less heat on the gate drive board and a gate-driver power supply smaller than would otherwise be needed. Another way a GDIC can shrink PCB area is by integrating several features that ordinarily might require several separate circuit elements. For example, the GDIC might incorporate the high-voltage Galvanic isolation. It might also incorporate less obvious features like post regulation of the gate power supply, so power supply outputs can be combined into fewer transformer cores, reducing the number of components that span the galvanic isolation barrier. If the ISENSE pin is available on the IGBT, there’s less need for Vce active clamping to address failure modes associated with high di/dt, high dv/ dt, gate-emitter and collector-emitter over voltages. Thus, there’s no need for active-clamping zeners and associated diodes.
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MEETING ISO 26262 ASIL-D METRICS ISO 26262 safety compliance is an emerging standard that will likely be required for most EV inverter systems. ISO 26262 entails a strict design and documentation process that ensures products meet stringent safety demands. A GDIC can offer features that simplify the process of meeting safety compliance specs. These features include safe SPI (serial peripheral interface) configurable mode, framing error detection, IGBT on/off state validation, continuous monitoring of power supplies, and interface pins for low-side and highside safety logic. Pins such as AMUXIN and AOUT provide a duty cycleencoded signal to represent the IGBT temperature, GDIC temperature, gate supply voltage, and an axillary AUMXIN monitor. This feature eliminates the need for external feedback from the HV domain. The final requirement for an ideal gate driver IC is design flexibility and reuse. The type, generation, size, pinout and features of IGBTs vary widely. So, the response of the GDIC must be tuned to optimize the performance of a given IGBT. By including an SPI interface, extensive programmability of the GDIC allows optimization of IGBT operation regardless of the specific IGBT chosen. The GDIC can set features such as short-circuit and over-current levels, over-temperature
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warning and over temperature shutdown, gate supply under-voltage shutdown threshold and filters. Moreover, the GDIC can help realize PWM deadtime, different operating modes, Vge monitoring, two-level turn-off voltage and time control. NXP offers an advanced gate driver for highvoltage power IGBTs which integrates the high-voltage isolator. The MC33GD3100 device offers current and temperature-sense features, including integrated current and temperature-monitoring outputs, that work directly with the new high-current-density IGBT modules. This gate driver provides quick detection of an over-current event as is necessary to minimize stress on IGBTs resulting from short circuits. Its integrated high-voltage electrical isolation between the low-voltage drive electronics and the high-voltage power electronics allow for communication between isolated and non-isolated domains. Its protection and diagnostics features are programmable, and it is compatible with fail-safe management from either the lowvoltage or high-voltage domains.
On the MC33GD3100, the SPI interface for ASIL C/D monitoring and reporting provides detailed fault and status data to meet ISO 26262 requirements. The GD3100 is also compatible with IGBTs that do not have a current and temperature sense pin; in this case it will monitor a collector-emitter voltage and provide desaturation threshold shutdown and clamping. All in all, the gate driver remains a key element connecting the power domain to the control domain and helps to drive higher system integration and lower design costs.
REFERENCES NXP www.NXP.com/smartpowerdrivers
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E L E C T R O L Y T I C C A P A C I TO R S
Electrolytic capacitors go svelte MARIO DIPIETRO, FLATPACK DIV., CORNELL DUBILIER ELECTRONICS, INC.
The traditional cylindrical packages that have characterized electrolytic caps are giving way to flat packs optimized for small spaces.
The
first reported developments of electrolytic “condensers” were in the 1880s, with significant advances in the mid 1920s. In 1948, etched-foil technology developed by Illinois Condenser (today’s Illinois Capacitor) and Alcoa allowed higher capacitance values in much smaller case sizes. Over the years, there have been incremental improvements, including new higher-gain anode etched-foil designs, higher-performance electrolytes, better construction technology and in-line QC testing. These updates have been responsible for size reductions, higher CV (capacitance – voltage) ratings for equivalent package sizes, lower ESR (equivalent series resistance), less power loss for the same amount of ripple current, and higher temperature ratings. The introduction of supercapacitors created a whole new product design paradigm. Supercaps are a form of electrolytic which offer massive storage capabilities at relatively low voltage ratings. In some applications, supercapacitors have even replaced batteries and have the advantages of quick charging and no loss of performance with each charge. Over the years, cylindrical form factors have prevailed, even at the surfacemount size level. Cylindrical package designs are easy to manufacture and versatile. However, it’s tough to significantly reduce the size of electrolytics. The reason becomes evident from a dissection of a typical electrolytic. It turns out that packaging materials (such as case, spacers, gaskets, etc.) take up about 60% of the space. There are also different designs for cylindrical aluminum electrolytics, as can be seen by comparing standard radial-leaded capacitors to radial snap-in types. The snap-ins, or snap mounts, provide a stronger mechanical connection to the PC board, and more robust internal construction, but they require vertical mounting. Thus they are unsuited to low-profile applications. Despite these differences, packaging elements take up a significant amount of the volume in both designs.
The CDE MLSH Flatpack aluminum electrolytic capacitor has a glass-to-metal hermetic seal and targets the mil/aero markets.
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The smaller the capacitor, the lower the energy density. A few examples illustrate the effect. Here, energy density is given in joules per cubic centimeter.
When cylindrical-style aluminum electrolytic capacitors are reduced in size, it becomes especially difficult to realize high energy densities because packaging occupies a higher percentage of the volume. Until now, applications requiring profile heights of under 10 mm necessitated creating banks of small capacitors. This approach takes up a lot of PCB area for both the component footprints and space in between. The latest electrolytic gamechanger came when Cornell Dubilier began offering flat electrolytic capacitors in the 1990s. At first, these devices were high-value components, designed exclusively for Mil/Aero and other high-reliability applications. The newest generation of flat aluminum electrolytic capacitors have retained many of those attributes
A comparison of aboard-mounted 5,800µF 35 Vdc THA/THAS capacitor and equivalent arrays of SMT and radial aluminum electrolytic capacitance arrays
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but with better economics. They are also lightweight and highly costcompetitive with more established high-performance technologies. Aluminum electrolytic capacitors can provide high capacity, low impedance and high-voltage ratings. Consequently, they offer benefits that are difficult to match with other capacitor dielectric materials. Flat electrolytics retain those qualities but can be designed for lower profiles, higher energy density and longer life. The idea of a high CV flat aluminum electrolytic fits well with board-level component trends. Flatpack technology manages to lower the component height by flattening the windings. Greater space efficiency is possible by placing those windings inside of a laser-welded, aluminum case. The differences in construction raise the cost of an individual Flatpack component, but the total cost of the bulk-storage capacitance is lower. This is because the high-energy density of the Flatpack makes it possible for one device to replace banks of cylindrical aluminum electrolytics. The Mil/aero versions of the Flatpack are designed to handle high vibration, up to 80 g, and some have true hermetic, glass-to-metal seals that prevent dry-out. These features, along with their high capacitance retention at -55°C, make them good candidates for military and aerospace applications. In particular, they are excellent options for replacing banks of costly wet tantalum capacitors.
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ADVANTAGES OF FLAT ELECTROLYTIC CAPACITORS Flat aluminum electrolytics are unlikely to ever fully replace their cylindrical counterparts. But flat electrolytics are good alternatives to banks of smaller capacitors when there is a need for low profiles, light weight, and high performance. Owing to its high-energy density and low profile, a single Flatpack can replace arrays of SMT, radial or axial aluminum electrolytic capacitors. This reduction in part count also improves system reliability. Of course, each capacitor eliminated also eliminates two PCB connection points that can be potential sources of failure. There are now two new less expensive executions of Flatpack technology. The 85°C THA Series Thinpack capacitors are only 8.2 mm high, and the 105°C-rated THAS version has a profile of 9 mm. This height is comparable to that of V-chip electrolytics and board-mounted axials, yet the device offers much higher bulk-storage capability. Both kinds of devices include a special valve to vent hydrogen gas, thereby reducing potential swelling that could arise if the internal pressure is not relieved. To work at 105°C, the THAS Series includes a 0.38-mm stainless steel sleeve which prevents the package from expanding at elevated temperatures. Potential applications include any circuits that require flat, high-capacitance bulk storage and filtering. This includes tablets, laptops, instrumentation,
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E L E C T R O L Y T I C C A P A C I TO R S
The CDE MLSH Flatpack aluminum electrolytic capacitor has a glass-to-metal hermetic seal and targets the mil/aero markets.
commercial-grade LED driver modules, compact power supplies, drones and RPVs, set-top boxes, 1U rack-mounted devices, and others. Key performance specifications include up to a 3,000-hour life at 105 °C without derating (THAS), and an energy density of up to 0.9J/cc. The need for low-profile, small devices will push manufacturers toward devising thinner and thinner capacitors, with even higher energy density. Some supercapacitors now sit in thin packages, so it’s reasonable to expect to see some electrolytics in equally thin profiles. Applications likely to benefit from flat-capacitors include power applications requiring high capacitance, bulk storage, and filtering of a dc supply. In power circuitry, smaller often means hotter, so performance at higher temperatures is also becoming more important. Flat electrolytic capacitors can bring more flexibility in the design of end products. And as this technology rapidly expands, economics and performance will continue to improve. Several manufacturers are currently developing technology that will squeeze package size even further. Thin supercapacitors are an indication of the trend. Some are in battery-style flat packages that are just a few millimeters thick. Advances in manufacturing technology will allow similar packaging for aluminum electrolytics. Besides being much smaller and lighter, capacitors will be made to fit available off-board spaces. All these component advances are opportunities for engineers to rethink what is possible and to find ways of making generational changes in their company’s end products.
REFERENCES Cornell Dubilier Electronics, Inc. www.cde.com
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POWER ELECTRONICS HANDBOOK
Five challenges in power supply design Sometimes the best way of handling
ALEX K ARAPETIAN, ACOPIAN TECHNICAL CO.
specialized power needs is through custom equipment optimized for the situation at hand.
Electronic
system designers, engineers, program managers and original equipment manufacturers (OEMs) must address numerous challenges in the selection or development of an appropriate power supply for their systems. In some cases, the best approach is to adapt an existing power supply from a different application. In other cases, it may be necessary to develop a new or custom power supply. To understand how a custom approach to power sources can be helpful, it is useful to examine five major challenges faced in the design of power supplies. Older systems that have not been updated for several years tend to use older-generation power supply technology with larger form factors. Depending upon the physical requirements of an application, power supply designers have had to become increasingly creative to adapt to changing space demands. We have reached a point where “off-the-shelf” supplies may be inadequate. For example, in one case a leading auto manufacturer needed a linearly regulated rack-mounting power system for a high-speed test data acquisition console. The system had to be custom-designed to fit in the 3U space that was available while operating at no more than 55°C. Customization included chassis slides modified to fit a special cabinet, and top and bottom covers.
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NON-STANDARD AC AND DC INPUTS — Today’s world is increasingly run by microprocessors that are sensitive to even small voltage fluctuations. Many power problems originate in the commercial power grid, which, with its thousands of An example of a custommiles of transmission lines, is subject designed power supply, to numerous power disturbances related configured to handle to weather conditions, equipment failure, traffic specialized requirements of accidents, and switching problems. Some of these problems include transients, interruptions, specific applications. voltage fluctuations and frequency variations. A robust power supply must reliably operate through input power disturbances to ensure the reliable operation of the systems it supports. While most applications still use 115 or 230-V inputs, new applications may call for special ac and dc input voltages, and power supply companies are adapting to meet these changing requirements. For example, in one case an industrial firm needed a redundant power supply system that accepted inputs from two separate power sources, one a battery backup. The use of the redundant inputs would ensure uninterrupted production on an automated assembly line. The solution came in the form of custom redundant supplies built with both 24-Vdc and 125-Vdc inputs and included dual voltmeters and ammeters. NON-STANDARD OUTPUT VOLTAGES — Electronic components and systems that operate at “standard” or “generic” voltages (e.g., 5, 12, 24, and 48 V) may be inefficient because the power that is not used produces extra heat in the system, which must be dissipated to keep the supply within its operating temperature range. Modern electronic components may need
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to operate at non-standard voltages to maintain high system efficiency while taking up a small amount of space. This requirement poses problems when off-the shelf power supplies aren’t feasible. For example, a company specializing in optoacoustic imaging needed a custom-designed rack-mount power system with many specific physical attributes. It had to have dc outputs providing 0.5 A at 5 V, 10 A at 6 V, and 25 A at 6 V. The supply also needed a 230-Vac IEC output to power a second rackmounted power supply. Additionally, all inputs, outputs, and controls had to reside discreetly on the rear panel, leaving the front panel completely blank. The supply needed provisions for airflow to cool both the custom supply and the second rack-mounting supply. But because of the existing setup, the cooling fan had to pull air in from the bottom of the unit. RUGGEDIZATION — Systems exposed to extreme temperature environments or high vibration often require environmental qualification in various, and often harsh, environments. Power supplies for the armed services must be built to specific standards. Many older systems don’t use components qualified for such environments. For example, in one case a major university research lab needed a redundant power source designed to handle extremes in temperature, elevation, and vibration. The application called for a custom power source that included features such as four sets of individually fused 48-V, 2.5-A dc outputs each with an output-fail LED, cascaded alarm contacts with audible alarms for each output, an alarm disable/timer switch, and a switchable 115/230 Vac input. COMPONENT OBSOLESCENCE— Short consumer product life cycles make components become obsolete quickly. For markets with long life cycles, component obsolescence becomes a critical issue, especially when OEMs are required to maintain hardware in the field. Many older electronic systems may have power supplies that are obsolete or whose manufacturer is no longer in business. When these systems fail, users must find alternative power supplies that meet the form, fit and function of the older system. If such supplies aren’t readily available, it may be necessary to redesign and recertify a new power supply.
Modular redundant power systems contain two identical power supplies with their outputs interconnected through a diode switching arrangement that will detect any fault condition, isolate the output of the defective supply from the redundant system output, and pass only the output of the other supply, with no interruption of the redundant system output power during the transition. This allows a defective power supply to be rapidly and safely changed while the modular redundant system continues to furnish uninterrupted power to the load. Redundancy can also apply to inputs. Here, two isolated sets of ac input connections permit using two independent sources of input power. By connecting a battery-backup power source and/or a second line from the power utility, output power will be maintained without interruption even when the main power fails.
In one case, the original manufacturer of a power system at an industrial firm had ceased operation. The dc-dc power supplies powered alarm systems at multiple facilities. To duplicate these supplies, engineers had to get photos of the existing supplies and copies of the literature that originally accompanied them. This approach enabled engineers to refine specifications prior to designing replacements. Features of these supplies included a 125-V dc input, outputs at 5.6-V and 4 A, 12 V and 15 A, and 125 V at 500 mA, provided through six sets of output terminals. CUSTOMIZATION — Specialized needs have made manufacturers more aware of power supply customization. An example is that of a leading manufacturer in the medical device field. It needed a special rack-mount power system that would sit in a dark room used for developing X-ray images. The equipment that generated and managed X-Ray data required reliable, consistent and clean power. This meant that in addition to standard power system requirements like multiple outputs and failure alarms, all LED indicators and digital meter lighting had to be capable of being switched off while the power system was still in use. And the power supplies had to provide consistently clean and reliable power within a tight tolerance of output current. The resulting custom design employed linear power supply modules and toggle switches on the front panel for individual control of LED Acopian Technical Co. www.acopian.com indicators and digital meter backlighting.
REFERENCES
Some power supplies available as standard products have a wide variety of options that allow a great deal of customization. An example is the Infinity series that has optional features that include four different overvoltage protection methods, IEC ac input connector options, four different ac input voltage options, power switch options, voltage output adjust and current limit adjust options, and many more.
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POWER ELECTRONICS HANDBOOK
Bearing trade-offs for cooling fans The life of most fans is set by the shaft
JEFF SMOOT, CUI INC.
bearings. Simple comparisons show why different bearing technologies can exhibit widely varying MTBF figures.
Cooling
fans are critical to many of the devices we rely on every day. A key criterion that affects fan life is the shaft bearing. This crucial element ensures the rotor can turn smoothly. As well as ensuring the rotor can make a significant number of rotations, the bearing may also be required to operate at different orientations and be able to withstand bumps and drops. Fan motor bearings are typically of either the sleeve or ball bearing variety. Both have their pros and cons, and designers are often forced into trade-offs when choosing one or the other. The sleeve bearing is the simpler and cheaper of the two. In this design, the central shaft pin spins inside a cylindrical sleeve with oil lubricating the bearing. The sleeve is responsible for holding the rotor in the correct position relative to the motor stator, ensuring the distance between the two remains constant. As well as costing less than ball bearings, sleeve bearings are typically more impact-resistant. However, the design has its disadvantages. For one thing, the gap between the inside of the sleeve and the shaft must be as small as possible to keep rotor wobble to a minimum. However, the tighter the sleeve, the more friction to be overcome when the rotor starts spinning. Consequently, sleeve bearings can be slower to start and need more energy to operate. A further friction-related issue with sleeve bearings is caused, ironically, by the oil rings and Mylar washers at either end of the bearing bore. These retain the lubricant needed to keep the shaft spinning smoothly and quietly, but their presence adds friction. They also trap some of the gas created by rotational friction. When this gas can’t escape, it solidifies into particles of nitride, which clog the
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bearing, hamper the shaft’s rotation, and ultimately shorten its life. The other big drawback of basic sleeve bearings is a consequence of the sleeve having full responsibility for holding the rotor in position: The full weight of the rotor rests on the inside of the bearing sleeve. As the rotor turns, gravity makes the shaft gradually wear away the inside of the sleeve. If your fan is always operating in the same position, your sleeve will develop an oval shape that can cause additional noise and rotor wobble.
SLEEVE BEARING
In this cross section of a fan with a sleeve bearing, fan rotation direction is into and out of the page. The gap between the inside of the sleeve and the shaft is kept small to minimize rotor wobble. Oil rings and Mylar washers are at either end of the bearing bore to retain lubricant. The gap, the rings, and the washers all contribute to friction.
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BEARING TRADE-OFFS
In this cross section
BALL BEARING
of a fan with a ball bearing, fan rotation direction is into and out of the page. Ball bearings put less friction on the rotor shaft than do sleeve bearings, and springs between the two ball bearings help offset any tilt caused by the weight of the rotor. But ball bearings are also noisier, more expensive, and less resilient than sleeve bearings.
Alternatively, if your fan will operate at multiple angles, the inside of the bearing will wear in different directions, resulting in an uneven shape that makes these noise and wobble issues worse. In the long term, all this wear shortens the life of the bearing and potentially that of the whole fan unit. So, while the sleeve bearing is both inexpensive and robust, these inherent drawbacks mean designers often look for alternatives. The most common of these is the ball bearing. Ball bearings consist of a ring of steel balls around the rotor shaft. When used in fan motors, you’ll typically find a pair of them on the shaft, separated by a ring of springs. When it comes to fans, this approach has several advantages over sleeve bearings. First, ball bearings reduce the amount of friction to be overcome when the fan starts and operates. Second, the springs between the two ball bearings help offset any tilt due to the weight of the rotor. By extension, the resulting reduced bearing wear typically brings a much higher mean time between failures (MTBF) than available with a sleeve bearing. Despite these pros, ball bearings have their cons. The fact that they can be used at any angle makes them seem more attractive than sleeve bearings for portable devices. However, ball bearings are less robust and must be treated with greater care. Ball bearings are also noisier than sleeves, while their greater complexity and component count means they’re pricier as well. Because both sleeve and ball bearings force compromises, CUI has developed a new type of fan that bridges the gap between traditional ball- and sleeve-
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bearing-based designs. Known as the omniCOOL system, this new fan design uses a magnetic structure to balance the rotor, in combination with an enhanced sleeve bearing. The rotor in an omniCOOL system operates like a spinning top that never falls over and can operate at any angle. The tip of the shaft works like the point on the spinning top, held in place by a supporting cap. The magnetic structure sits in front of the rotor and uniformly attracts it around its entire circumference, balancing the rotor regardless of the angle at which the fan operates. Consequently, the inside of the bearing needn’t support the weight of the rotor – this is instead borne by the magnetic flux and supporting cap. The omniCOOL system reduces or removes many of the drawbacks of traditional sleeve or ball bearings. For example, the magnetic structure actively balancing the rotor minimizes the tilt and wobble issues common with standard sleeve bearings. And because the shaft doesn’t rest against the inside of the bearing, friction between the two is dramatically lower than with a traditional sleeve bearing. Experienced designers will know that a magnetic structure of this kind could be applied to a traditional sleeve or ball bearing. However, this measure on its own wouldn’t be enough to overcome all the challenges mentioned earlier. And this is where the enhanced sleeve bearing of the omniCOOL system comes in.
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ONMICOOL SYSTEM Again, fan rotation direction is into and out of the page on this fan motor with the omniCOOL system. The tip of the rotor shaft is held in place by a supporting cap. A magnetic structure sits in front of the rotor and uniformly attracts it around its entire circumference, so the inside of the bearing doesn’t have to support the weight of the rotor.
The sleeve used in the omniCOOL system is specially hardened to resist against abrasion and heat. This enables operation at up to 90°C while traditional sleeve bearings can typically only withstand up to 70°C. The hardened sleeve and reduced abrasion (thanks to the magnetic structure balancing the rotor) also dramatically extend bearing life – test results have shown an omniCOOL system lasting over three times longer than a standard sleeve bearing when operated at 70°C, rising to five-and-a-half times longer at 20°C. Another advantage of omniCOOL systems is they need no oil rings and Mylar washers. The magnetic structure minimizes the chance of the shaft rubbing against the inside of the bearing, so oil rings and washers are no longer needed. This further reduces friction and provides clear space at either end of the bearing for the escape of any gas produced by rotational friction. It also reduces the cost and complexity of the overall design to speed manufacturing and quality assurance, compared to more complex setups.
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REFERENCES CUI omniCOOL fans www.cui.com/advanced-sleeve-bearing-dc-fans
154 Hobart St., Hackensack, NJ 07601 USA • +1.201.343.8983 • main@masterbond.com
www.masterbond.com
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DESIGNING WITH BRICKS
Designing with bricks – baseplate-cooled converter modules GARY BOCOCK, XP POWER
Power converter modules can simplify equipment design, but engineers may still need to look at EMC emissions and input-surge filtering.
Baseplate
cooled power converter modules, also known as bricks, provide building-blocks for integrating power conversion into end equipment. These highdensity power modules find use in outdoor sealed enclosures, transportation and defense applications where their rugged construction and conduction cooling come in handy. They also are widely applied where high-density forced-air-cooling and compact size bring benefits. Bricks provide the building blocks for low-risk bespoke power supplies. These supplies are often designed by end-equipment designers using support and application notes from the manufacturer. But module manufacturers themselves use these modules as bases for custom supplies that have low development costs, and which can be brought to market quickly. However, baseplate-cooled converters are component-level rather than drop-in products. They generally require additional design work and components to handle electrical safety, thermal management and electromagnetic compatibility (EMC). They target both dc and ac input applications. Their typical input ranges are designed to cover battery and dc vehicle supplies as well as higher voltage, rectified or power factor corrected (PFC) ac supplies. The industry has developed standard sizes for these modules termed quarter bricks, half bricks and full bricks. Power ratings up to 600 or 700 W can be realized in a standard 2:1 input full brick. However, power density drops as the input range widens to 4:1, 8:1 or even 12:1. The trend toward wider input ranges has come out of efforts to accommodate several different battery chemistries and from attempts to standardize system design for multiple power platforms.
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Baseplate-cooled converters with ac input are also available providing ac to low-voltage dc conversion, or as PFC modules with high-voltage outputs (usually around 400 Vdc) to drive high-input-voltage dc/dc bricks. Most of these ac-input products also require additional design and components, including high-voltage electrolytic capacitors and components for EMC. However, a few do not. An example is the ASB110, 110-W complete ac/dc fullbrick supply from XP Power, which requires only thermal management and includes all other parts. THERMAL MANAGEMENT Thermal management is a key for power bricks designed to be baseplatecooled. The brick is configured so power-dissipating components, such as the power semiconductors and transformer, are thermally bonded to the baseplate. The baseplate temperature must always stay below a maximum operating temperature. The thermal resistance of the cooling scheme must be such that the module stays below the maximum temperature spec while powering the load. The power dissipated (in watts) can be determined from the module efficiency specification under the worst-case operating conditions. But it is
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MODULE EFFICIENCY
TYPICAL POWER MODULE EFFICIENCY VS. LOAD
PERCENT OF LOAD An example of the variation in efficiency with input voltage and load.
important to consider the both the operating load and lowest input voltage applied rather than the data sheet headline efficiency. Once the efficiency at the worst-case operating point is established, the waste power to be dissipated as heat is calculated as
where Ef is efficiency in percent, Pout is output power in watts. A simple equation, θ = ∆T /Q, determines the thermal resistance required for operation. Here ∆T is defined as the difference in °C between two reference points; θ is thermal resistance in °C/W, and Q is the heat flux or power in Watts passing through the two points. The thermal resistance, θcs, from the case to heatsink is typically 0.1 ⁰C/W when using a thermal pad or grease. Use of this equation allows the calculation of junction temperatures using a thermal circuit analogous to that for an electrical circuit. The thermal resistances can be treated mathematically the same as electrical resistances. Thermal resistance to the flow of heat from the power module to the ambient air surrounding the package consists of two thermal resistances: that from the case to the heatsink, and that from the heatsink to the ambient interfaces. The two resistances can be summed to give an overall thermal resistance from power module to ambient:
where TC = maximum power supply temperature; TA = ambient temperature, PD = power dissipation, and θCA = caseto-ambient thermal resistance. The thermal resistance of the heatsink-to-ambient depends heavily on available airflow; in convection-cooled applications the heatsink will be much larger than in a comparable power system with forced air or liquid cooling. When using multiple bricks connected to a common heatsink or cold wall, the sum of the dissipated power from each brick in the system, under worst-case conditions, determines the overall maximum thermal resistance allowable. Heat sinks and other thermal management measures aren’t the only components baseplate-cooled modules need before they switch on for the first time. Other additions include those for reverse polarity protection and control of noise emissions, as well as for protection against spikes and surges. Noise mitigation typically takes the form of a capacitor network, inductors, and surge-suppressing components. And fusing is a must to head off catastrophic failures from shortcircuiting the supply. The power module data sheet and application notes will specify the values of the components required. It is up to the design engineer to implement them, following good design practices for any creepage and clearance requirements, and minimizing parasitic inductance for EMC compliance. A nearby circuit diagram illustrates typical components found in power supply circuits to handle noise, filtering, and other issues. Here, FS1 protects against an input short-circuit failure. L1, C1 and C2 form a pi filter to mitigate differential noise created by rapid changes in current through the power switching stage. Similarly, L2, C4 and C5 form a common-mode filter to mitigate noise created by the rapid changes in voltage in the power stage. C3 presents a low impedance source for the power converter switching current demand, and TVS1 is a bi-directional transient surge suppressor to protect against
TYPICAL THERMAL RESISTANCE PATH
A typical power brick and heatsink thermal model. The thermal resistance from heatsink to ambient, θ sa , and the
thermal resistance from case to heatsink, θ cs , are treated in a manner analogous to electrical resistors in series.
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DESIGNING WITH BRICKS
spikes and surges. C6 and C7 reduce output common mode noise. An additional differential filter may be added at the output for applications that require extremely low noise. In general, the decoupling capacitors (C4, C5, C6 and C7) should be as close as possible to the pins and chassis connection to the baseplate to keep the PCB traces as short as possible. The input electrolytic capacitor C3 and transient voltage suppressor TVS1 should sit physically close to the input pins of the module. And tracks beneath the power module should be avoided. A limited number of application-specific filter modules are available to handle the abnormal surges found in dc input transportation and defense applications. These generally also require a few additional components for full compliance. The near-by diagram shows the case where an input protection (DFS) module contains all the necessary active surge protection circuits and the filter inductors necessary for a 28-V nominal military vehicle supply. Here, C1 completes the differential filter stage. C3 and C4 make up the common-mode filter stage. C2 provides a low-impedance source for the MTC series dc/dc converter. A PFC module for an ac input power system requires similar EMC components, and a high-voltage (450 Vdc) electrolytic bulk capacitor (C6) is also necessary. The system hold-up or ridethrough requirements determine the value of the bulk capacitor. Some ac input solutions combine the PFC and dc/dc sections into one brick, with connections made available for the bulk capacitor. There are additional requirements for ac input systems regarding creepage and clearance distances between line and neutral, and between line and neutral and earth. These creepage and clearance distances are outlined in the end application’s relevant safety standard. Of course, the added inductors, capacitors, and other components must remain within their thermal limits. The electrolytic capacitors must exhibit a service life acceptable for the mission profile of the end equipment. Insulating thermal pads may help conduct heat away from the inductors to the equipment cold wall. Other components often mount on the reverse side of the PCB away from any higher-temperature parts. The best manufacturers of these high-density baseplatecooled brick converters provide application information and EMC data to support their products. They also maintain experienced applications engineers to support equipment designers during the design and compliance testing phases. And they typically offer pre-compliance testing against common industrial, communications, transportation and defense EMC standards. Module manufacturers also have seen myriad applicationspecific custom designs; they can pass on lessons learned from these invaluable experiences. All in all, having the module manufacturer design and produce a bespoke brick-based power system eliminates the need for additional design and testing resources so the OEM can focus on the core system design.
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TYPICAL FILTERING COMPONENTS FOR PFC PLUS POWER MODULE
A typical schematic for an ac input system containing a PFC module.
FILTERING COMPONENTS FOR INPUT PROTECTION (DSF) MODULE
A typical schematic for a dc input defense application including a DSF filter. Here D1 protects against a reversed polarity on the dc input.
TYPICAL FILTERING COMPONENTS FOR A POWER MODULE
A typical schematic for a dc input system that includes components for EMC and filtering.
REFERENCES XP Power www.xppower.com
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Measuring current with shunt resistors Here are a few tips for selecting
BERT WEISS, RUTRONIK
resistors that will accurately gauge current.
Today,
practically every control and monitoring circuit uses shunt-based current measurements as METAL-LAYER SHUNT, TYPICAL CROSS SECTION an alternative to sensors. To make these measurements accurately, it is useful to understand how shunts work. Because the method is categorized as a precision measuring technology, it should not be regarded as trivial. A shunt is a low-value resistor used to measure current - it is therefore also referred to as a current-sense resistor. The shunt typically connects in series so it carries the current of interest. A voltage measurement device then connects in parallel with the shunt. The current through the shunt generates a voltage drop that is measured. The current value is derived from Ohm’s law and the known resistance (I = V/R). To keep power loss - and thus heat development - to a minimum, shunts must have resistive values no higher than the milliohm range. Some are even below that. The advantage of this measuring With metal layer resistors, a resistive paste method is that it allows the quick detection is applied to a substrate and adjusted to and elimination of faults. Shunts are therefore particularly the desired value using laser trimming. interesting for safety-relevant applications where faults must be This results in an inhomogeneous structure detected. Furthermore, shunts deliver precise measurements that is trimmed to the rated value as a and thus enable the efficient control of drives or the monitoring meandering shape. of battery management systems. And shunt resistors are an excellent value for money. Shunts are basically suitable for any type of measuring application - be it direct or alternating current. Shunts are currently experiencing a boom, especially thanks to the rising
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number of condition measurements in vehicles -- engine and TYPICAL FOUR-WIRE CURRENT battery management, airbag control units, ABS, infotainment SHUNT CONFIGURATION systems, and so forth. Current-sense resistors are also becoming ever more widely used in industrial applications, medical technology, for regenerative energies, and for smart metering. Shunts are available as metal layer technology and in full-metal versions. Layer resistors are considerably less expensive but their resistance value changes with temperature to a greater degree than do the full-metal devices. The make-up of metal-layer Full-metal shunt resistors consist shunts also brings a notable of a homogeneous resistive disadvantage: With metal layer element so that no additional resistors, a paste is applied to a inductance develops. ceramic substrate and adjusted to the desired value using laser trimming. This results in an inhomogeneous structure that is trimmed to the rated value as a meandering shape. This meandering shape causes a series inductance, potentially Some shunts have four wires. Here, the current degrading current measurements. The voltage drop at flows through two connections and the voltage is the shunt, U, then follows the equation U = I x R – L(di/dt). measured at the other two. The voltage drop at the resistors can be determined using the internal Consequently, metal layer resistors are only worth considering Kelvin connections. if inductance is unimportant. Full-metal shunt resistors consist of a homogeneous resistive element so that no additional inductance develops. This quality is key in such high-precision applications as Some shunts have four wires. Here, the current flows medical technology or precision measurement devices. through two connections and the voltage is measured at Furthermore, these resistors are characterized by high the other two. The voltage drop at the resistors can be measurement accuracy and resistance to thermal shock. They determined using the internal Kelvin connections, so the are available in various sizes - including versions that are much resulting measuring errors can be eliminated. larger than standard chip resistors - and TK-values way below Four-wire shunts are used in two scenarios: First, 100 ppm/K. Full-metal resistors can operate with an output of when the line and contact resistance are relatively high up to 7 W at maximum temperatures of 275°C. They can have and, in contrast to the measured resistance, not negligible. resistive values up to the low single-digit milliohm range. Second, when the resistive value is below 10 mΩ. Because The optimum resistive value can be determined quite the resistive values of the conductors are also in the easily: The lowest measuring voltage that still gives sufficiently milliohm range, they must thus be accounted for. accurate results is divided by the lowest current value of the measuring range. There is a trend toward smaller shunts with higher outputs; also in wider use are customer-specific versions with special connection geometries and sizes. As shunt resistors are Rutronik Elektronische Bauelemente GmbH relatively expensive compared to other resistor technologies, www.rutronik.com they are available in small batch sizes and test samples.
REFERENCES
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TO U G H T E R M I N O L O G Y
Tough terminology: Why working voltage matters in safety approvals Organizations that test power circuits for safety have
DYLAN HOWES, MINMAX POWER, INC.
specific definitions of working voltage and other parameters that bear heavily on how products get safety certifications permitting their sale.
Safety
approval agencies have been evaluating Verein (TUV) grant certifications of compliance to both industrial and consumer products for products against safety standards specified by the well over a century. The need for approval by such agencies International Electrotechnical Commission (IEC) and/ has risen rapidly since the early 1920s. Today it would be or the International Organization for Standardization impossible to bring a product to mass market without first (ISO). Variations do exist between the conditions of obtaining the industry standard safety approvals. compliance from agency to agency for a single given For product design engineers, it is critical to consider standard, depending primarily on the geographical the implications of applicable safety approval standards early in the design TYPICAL EQUIVALENT CIRCUIT FOR process. This means understanding TRANSFORMER ISOLATION the safety approvals granted to components and subsystems. Navigating safety approval requirements can be cumbersome, and there are some common misconceptions associated with the standards that govern dc-dc power converters. Of particular importance are working voltages and how they affect some of the parameters to which safety standards apply. Approval agencies such as the Underwriters Laboratory (UL) and the Technischer Überwachungs-
Transformers can provide isolation between two sides of a circuit, but there is still a potential connection between the two sides. One way to model the connection is with an RC network. Both R and C have extremely high values.
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region in which the agency operates and the applicable legislation within that region. It is for this reason that compliance declarations are often given as the IEC or ISO standard number, prefixed with an abbreviation that indicates the region for which the standard has been approved i.e. EN60960-1 (EN stands for European Norm). DC-DC power converters are typically evaluated against IEC60950-1, the Standard for Safety of Information Technology Equipment (ITE), but can also be evaluated against additional, more specialized standards depending on the market of the end product. One such specialized standard is the standard for Safety of Medical Electrical Equipment, or IEC60601-1. This standard specifies much higher degrees of isolation than IEC60950-1. It does so to
semiconductor device such as a linear regulator and does not necessarily have any galvanic isolation from higher voltage, hazardous circuits, or from earth ground. FELV is the least stringent of the three ELV designations. A Protective ELV (PELV) must have a galvanic isolation from any nonELV circuit and must not become continuous with those higher voltage circuits via any single fault, apart from an earth ground fault. This means PELV circuits may contain a direct connection to protective earth ground. The most stringent ELV designation is the Safety ELV (SELV). Some standards refer to an SELV as a Separated ELV rather than a Safety ELV. An SELV is an ELV which cannot under any single fault condition become continuous with any non ELV circuit or with protective earth ground, such that even in the event of an earth ground fault, the ELV is maintained. SELVs are typically obtained through use of reinforced insulation. One major focus of most electrical safety standards is the isolation barrier between hazardous voltages (greater than approximately 42 Vac or 60 Vdc) and these SELV circuits. End-products must be designed such that a user cannot touch hazardous high voltages. In an isolated power supply, the input terminals are not electrically connected to the output terminals. Rather, energy transfers from the primary side of the converter
EXTRA-LOW-VOLTAGE CIRCUITS
protect patients who may be in continuous or temporary physical contact with an electrical device that is fed from a hazardous voltage source such as a 120-V or 230-V ac wall outlet. IMPORTANCE OF ISOLATION Many relevant safety standards specify criteria for a circuit to be considered an Extra-Low-Voltage (ELV) circuit. This designation indicates that the potential differences between conductors within the circuit do not exceed a certain value. This value may differ between standards but is typically around 42 Vac or 60 Vdc. These lower voltages greatly reduce the risk of electric shock. There are three commonly referred to types of ELVs. A Functional ELV (FELV) typically gets its ELV status via a
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to the secondary side via the magnetic field within a transformer. This electrical isolation will allow the secondary side of the converter to be classified as an SELV even if the primary side has an electrical connection to a hazardous voltage, under the condition that the isolation barrier can prevent arcing or tracking that would render the primary and secondary sides continuous. Accordingly, both IEC60950-1 and IEC60601-1 specify tests, along with pass/fail criteria, that determine a product’s ability to keep high voltages away from the SELV circuits that may reasonably touch users during normal operation, or during single fault conditions. The standards also specify acceptable levels of leakage current, the mains current that flows through the isolation barrier during standard operation of the device. DEFINING WORKING VOLTAGE Working voltage is a parameter that can be used to describe and qualify the standard operating voltages that a system or sub-system can or will see during normal use. In general, the working voltage of a given device is the highest voltage to which that device can be subjected continuously without beginning the process of dielectric breakdown, or otherwise sustaining damage.
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TO U G H T E R M I N O L O G Y
Current Sense Resistors We can also describe an electrical system’s working voltage as the highest voltage present within that system during normal operation. That is, any electrical device that gets its power from a standard ac wall outlet in North America could be said to have a working voltage of 120 Vac. Conversely, we might describe an electrolytic capacitor somewhere within the device, downstream of a dc switching regulator, to have a working voltage of 25 V, the highest rated voltage that that the capacitors dielectric material can withstand without breaking down. Safety standards utilize working voltage ratings to determine requirements on other parameters such as dielectric strength and creepage distance. Dielectric materials used for electrical insulation have large nominal electrical resistivities, typically on the order of tens or hundreds of tera-ohm-meters. These nominal resistivities, however, are ultimately a function of the electric field strength within the material – there is an electric field strength for which the dielectric material will experience a sudden and drastic drop in resistivity. Such an event will allow current to flow relatively unimpeded through the material. This phenomenon is known as dielectric breakdown. If an insulation barrier between a hazardous-voltage circuit and an SELV circuit breaks down, harmful currents can flow into the SELV circuit, posing serious health risks to anybody touching it. For this reason, safety standards specify requirements on the dielectric strength of the insulators that create these barriers. These requirements are verified during the safety agency approval process via a dielectric strength test, commonly known as a Hi-Pot (High Potential) test. An insulating material’s rated working voltage is used, in part, to determine the Hi-Pot test voltage and duration according to tables given in the safety standards. A particular standard should be referenced to determine exact Hi-Pot test values for a given insulation. But a useful rule of thumb is to assume a Hi-Pot test voltage of at least 1 kV greater than twice the rated working voltage as given in equation [1]:
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KOA Speer’s continually expanding line of Power Shunt Current Sense Resistors are the ideal solution to help you optimize your power system design. Our new Power Shunt Current Detectors are designed for DC to DC converters, inverters, batteries, motor controls, automotive modules, power supplies or any other power management application. KOA Speer Power Shunts Deliver: • High Power: up to 10W • Ultra Low Resistance: 0.5mΩ ~ 1mΩ • 4 Terminal, 2726 size - PSG4 2 Terminal, 3920 size - PSJ2 • Wide Temp Range: -65°C ~ +75°C
Check out the new products from the industry’s most award winning passive component supplier… KOA Speer. where VHi-Pot = Hi-Pot test voltage, Vw = rated working voltage. In reality, standards typically specify a somewhat less linear, and frequently more stringent, working-voltageto-dielectric-strength relationship. Recall that dielectric breakdown results from an excessive electric field strength. Electric field strength is a function of both the potential 2 • 2018
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difference between two conductors and the distance between them as given in equation [2]:
where d is the distance in meters between the conductors. Accordingly, it is not the resistivity of the dielectric material alone that determines its ability to separate a hazardous voltage circuit from an SELV circuit. The physical separation of the conductors and the thickness of the dielectric also factors in. It is for this reason that safety standards mandate minimum distances between the conductors of a hazardous voltage circuit, and an SELV circuit. These separation distances are known commonly as creepage and clearance distances. Creepage is the shortest distance between two conductors as measured across the surface of a dielectric material. Clearance is the shortest distance between two conductors as measured through air. A circuit’s nominal working voltage is used, in part, to determine these minimum spacing requirements. Other factors that are considered when determining minimum spacing requirements are the environment (i.e. the likelihood that conductive pollution will accumulate across the surface of an insulator) and the overvoltage category of the circuit (the likelihood of occurrence, and magnitude of transient voltages). Leakage current is another parameter that is important for evaluating an isolation barrier between a hazardousvoltage circuit and an SELV circuit. Two different values of leakage current are typically specified and evaluated during the safety approval process: earth leakage current and touch current. Earth leakage current is not typically an important parameter for dc-dc converters as they are downstream of the main ac-dc converter, and they are not frequently connected to earth ground.
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Touch current is a maximum measure of how much current flows from the primary side of a dc-dc converter to the secondary side at the insulator’s rated working voltage. A real insulator can be modeled as a resistor and a capacitor in parallel. The value of resistance R is determined by the insulator resistivity and physical size according to equation [3]:
where ρ is the dielectric resistivity, l is the thickness of the dielectric along the shortest path between the two conductors of interest, in meters, and A is the cross-sectional surface area of the insulator, in meters-squared. The value of the capacitor C in the model is determined by the material’s relative permittivity according to equation [4]:
where εr is the material’s relative electric permittivity, ε0 is the permittivity of free space, and A and l are as defined above. Using the equations [3] and [4], along with Ohm’s law, it can be shown that the current flowing through the complex impedance of an insulator between two isolated circuits can be described in terms of its working voltage and material properties as given in equation [5]:
where IL is the leakage current in Amperes, Vw is the rated working voltage, j is the square root of (-1), f is the frequency of voltage, and the other parameters are defined as above. In examining equation [5], one can observe that leakage currents are mitigated by insulators with large resistivities and by longer creepage distances. Conversely, leakage currents are worsened by
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higher relative electric permittivity, larger cross-sectional surface areas, voltages at higher frequencies, and of course, higher working voltages. Note that in the case of dc-dc regulators, f is zero. The second term in the numerator can be omitted leaving only a few material parameters of interest. The bottom line for leakage current is that higher system working voltages call for well-designed insulation barriers capable of keeping hazardous currents out of SELV circuits during normal operation. When designing a piece of equipment that will go through a safety approval process, designers must consider how the system working voltage and the rated working voltages of insulating materials affect the parameters that safety standards will test. A good first step is to select a dc-dc converter that meets and/or exceeds the dielectric strength, creepage, clearance, and leakage current requirements associated with the product’s working voltage. Though dc-dc converters are often used to convert relatively low voltages on the primary side, these seemingly low voltages may be galvanically connected to circuits at hazardous voltages in practical applications such as motor control systems or other offline designs. Use of a dc-dc converter with a high working voltage rating readily allows the secondary side of the converter to be considered a SELV circuit, regardless of any galvanic high voltage connections upstream in the system.
REFERENCES MINMAX Power, Inc., www.minmaxpower.com
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2/16/18 10:06 AM
H I G H - T E M P E R AT U R E
High-temperature, harshenvironment tantalum capacitor technologies CHRIS REYNOLDS, AVX CORP.
Solid-tantalum capacitors have evolved with better design, construction, and testing methods to handle emerging requirements for harsh, high-temperature applications that put a premium on reliability.
Wet
tantalum capacitors are a proven, mature TYPICAL CAPACITANCE vs. TEMPERATURE technology. They’ve long been employed in demanding, high-temperature applications because they have a variety of useful qualities. These include a nonsolid electrolyte capable of controlled self-healing, high bulk capacitance at high-voltages (e.g., up to 5,600 µF at 125 Vdc), high volumetric efficiency, excellent stability, good high-temperature performance, high-reliability and a long lifetime. In recent years, this technology has been further developed to handle harsh 200°C industrial applications. However, the trend in consumer, communications, medical markets has long been toward digital applications with low operating voltages (e.g., 6 –25 V) that could be handled by surface-mount capacitors. So, solid-tantalum chip capacitors up to 220 µF and rated to 200°C have come to be preferred over hermetically-sealed, axialleaded wet tantalum capacitors. The reasons: Solidtantalum chip capacitors are smaller, cost less, and deliver Capacitance vs. temperature for the AVX THJ Series tantalum capacitors vs. the the low-ESR and high-frequency response necessary for high-speed digital OxiCap NOS Series niobium oxide ceramic applications. capacitors, which are manufactured using Additionally, in recent years, solid-tantalum capacitors with hermetic, the same process as tantalum capacitors surface-mount device (SMD) packaging became available. This packaging and exhibit electrical parameters similar allows the internal element of a solid tantalum chip capacitor to operate in to general, low-ESR tantalum capacitors. an inert gas. This environment helps resist moisture ingress and enables operation at temperatures up to 230°C with higher capacitance and voltage ranges (up to 330 µF and 63 Vdc). Hermetically sealed, high-temperature,
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POWER ELECTRONICS HANDBOOK MAXIMUM CATEGORY VOLTAGE vs. TEMPERATURE
Voltage derating vs. temperature for the AVX THJ Series tantalum capacitors vs. the OxiCap NOS Series niobium oxide ceramic capacitors.
solid-tantalum SMD capacitors also stand up to harsh mechanical shock and vibration. They are available with a range of termination finishes designed for compatibility with high-temperature PCB and hybrid circuit assembly, high-melting-point (HMP) solder, epoxy, and wire bonding processes. Tantalum capacitors are one of just a few capacitor technologies that can operate reliably at temperatures above 175°C. Basic capacitors consist of a pair of conductive or semiconductive plates separated by an insulating dielectric. The dielectric stores charge when voltage is applied, effectively blocking dc and enabling the transmission of any ac signal. Tantalum capacitors are a subset of electrolytic capacitors. These are polar devices in which one plate is maintained at a positive potential and the other at a negative potential. Solid tantalum capacitors have a tantalum positive plate with an insulating film of tantalum pentoxide, which acts as the dielectric, on the surface. A negative plate (i.e., a counter electrode or cathode) is made of manganese dioxide or a
LEAK AGE CURRENT vs. TEMPERATURE
Leakage current vs. temperature for solid tantalum capacitors
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2 • 2018
conductive polymer. In wet tantalum capacitors, the negative plate is a high-surface-area tantalum cathode that includes an acid electrolyte. The top two tantalum technologies for high-temperature applications are solid tantalum capacitors with a manganese dioxide cathode in a molded SMD or hermetic ceramic package, and wet tantalum capacitors with a hermetic metal can construction and an axial lead. The cathode type has the greatest impact on a capacitor’s frequency qualities and determines the applications to which it is best suited. For example, solid tantalum exhibits a better frequency response for filtering applications, and wet tantalum exhibits higher bulk dc capacitance and higher voltage ratings. TANTALUM CAPACITOR PERFORMANCE Tantalum capacitors are known for their reliability, ruggedness, high volumetric efficiency, and parametric stability. Standard tantalum chip capacitors are rated for operating temperatures AVX THH 230°C Hermetic Series spanning -55 to high-temperature SMD tantalum +125°C. This range chip capacitors. easily handles most consumer and incabin automotive electronics. Enhanced, professionalseries tantalum chip capacitors can be made to comply with AEC-Q200 automotive industry specifications and to withstand continuous operation at temperatures up to 175°C with a base reliability of 0.5% per 1,000 hours. Advanced, high-temperature, molded tantalum-chip capacitors capable of operating continuously at up to 200°C are also available. Beyond this are two recently introduced series: an extension to existing high-temperature axial-leaded wet electrolytics and a hermetically sealed SMD version. Both can operate as high as 230°C. A look at the temperature coefficients of tantalum capacitors reveals they gain in capacitance value at higher temperatures. Wet tantalum capacitors can realize
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2/16/18 10:09 AM
When it comes to power inductors, we’re spreading things a little thin (actual height)
In fact, with hundreds of options under 1.2 mm high, selecting the perfect low-profile power inductor is no tall order! Let’s face it, thin is in. From smart phones and wearables to all types of portable devices, you face constant pressure to pack more performance into the thinnest packages possible. To help, we continue to expand our line of mini, low-profile power inductors with footprints as small as 1.14 x 0.635 mm and heights as low as 0.50 mm! Select inductance values from 0.018 to
3300 µH and current ratings up to 20 Amps. Get the skinny on all our low-profile power inductors, including our ultra-low loss XEL4012 Series with inductance values that have been fully optimized for high frequency applications over 5 MHz. Visit coilcraft.com/lowprofile today! ®
WWW.COILCRAFT.COM
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POWER ELECTRONICS HANDBOOK
AVX TWA-X Series 230°C hightemperature, axial-leaded wet tantalum electrolytic capacitors.
even higher capacitance at high temperatures. The capacitance tolerance limit at the 25°C specification for solid, hermetically sealed, hightemperature tantalum capacitors is ±20%, while wet electrolytic capacitors are made with tolerance bounds of ±10% and ±20%. Tantalum and niobium-oxide ceramic capacitor technologies also have different temperature limits for capacitance. Solid, hermetically sealed tantalum chip capacitors exhibit narrow capacitance over their operating range, dropping up to 20% at -55°C, rising by 20% at 85°C, and holding to a maximum 30% increase from 150°C to 230°C. Wet tantalum capacitors display a much wider variation over their operating temperature range. The level of variation depends on their size and capacitance/voltage (CV) rating. The capacitance of wet tantalum capacitors can drop by 20% to 85% at -55°C and rise from 12% to 80% at 125°C. In addition, tantalum capacitors exhibit an applied voltage vs. temperature relationship. At high temperatures, the voltage ratings specified at ambient temperatures must be derated to avoid diminished
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reliability. The category voltage — the maximum voltage at which the part can operate up to 230°C — for solid, hermetically sealed, high-temperature capacitors is high for solid tantalum technology: 50% of its rated voltage. This property can be advantageous at high temperatures. The higher reliability resulting from operating at a reduced voltage can more than compensate for the lower reliability resulting from operating at high temperature. Tantalum capacitor direct current leakage (DCL) also depends on temperature, rising by a factor of 10 between 25 and 85°C. DCL arises from parallel resistance paths in the dielectric and results in the slow discharge of the capacitor. In tantalum capacitors, leakage current rises linearly with temperature, which increases energy loss. The DCL specification limit for 125°C operation for solid, hermetically sealed, high-temperature tantalum capacitors is 12.5 times the initial limit. For about 50 years, wet tantalum capacitors packaged in a hermetically sealed, (typically) tantalum metal housing have been available in hightemperature designs suitable for operating up to 200°C with a category
2 • 2018
voltage equaling 60% of the rated voltage at room temperature (0.6Vr). More recently, material advances in powder, silver, molding resin, and other materials have also allowed solid tantalum chip capacitors in molded bodies to build up a track record in 200°C applications. It has been challenging to boost the operating temperature for tantalum chip capacitors beyond 200°C because this temperature exceeds the glass transient temperature of the epoxy materials used in the capacitor body construction. Design engineers finally devised a reliable, SMD capacitor rated for 230°C by housing solid tantalum capacitor elements in hermetically sealed ceramic packages. These high-temperature SMD tantalum capacitors enclose the capacitor element in a hermetically sealed ceramic housing filled with inert nitrogen gas to create an inner atmosphere that inhibits the oxidation of the solid tantalum electrolyte. This design has performed well during both high-temperature operating life testing (2,000 hours at 230°C and 0.5Vr and 10,000 hours at 200°C and 0.5Vr) and moisture resistance testing (1,000 hours at 85°C and 85%RH, Vr). Solid, hermetically sealed tantalum chip capacitors are also rugged, satisfying mechanical shock and vibration specifications of 100g shock and 20g vibration. Available ceramic housing sizes include CTC-21D (“9”) and “I” cases with L-shaped or flexible J-lead terminations for applications subject to extreme shock and vibration or undertab (facedown) terminations for applications in which minimal footprints take precedence. The smallest case size
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2/20/18 2:16 PM
H I G H - T E M P E R AT U R E
plasticity that can degrade PCB measures 11 (±0.2) x 6 (±0.2) x mounting. As such, it is important that 2.5 mm (L x W x H), while the the HMP solder material functions well largest measures 12.10 x 12.5 x above the operational temperature. 6.5 mm (L x W x H). Ditto for high-temperature epoxy The capacitor elements in used for mounting as well as these packages provide high mechanical reliability via its elastic capacitance ratings (up to 100 properties. µF) over a wide voltage range Molded SMD, hermetic SMD, and (16–63 Vdc). Their capacitance AVX TWD 125°C High-Temperature Maximum axial-leaded, hermetically sealed wet is stable over the range of rated Capacitance (HTMC) Series axial-leaded, tantalum capacitors all have different operating temperatures (-55 to hermetically sealed wet tantalum electrolytic capacitance and voltage range +230°C). In that the capacitance capacitors in a standard T4 case. capabilities. Each are better suited of tantalum capacitors actually to certain applications than others. rises with temperature, these When their performance capabilities overlap, the cathode capacitors can lead to significant size reductions and lower type, which also greatly affects frequency performance, will component counts in a variety of critical, high-temperature, help determine the best applications for each technology. high-reliability electronic systems. In downhole electronics, high-temperature is usually New axial-leaded, hermetically sealed wet tantalum classified as 150°C and above. Temperatures of 150°C to electrolytic capacitors are also capable of 230°C maximum 175°C were long considered the maximum typical for drilling operating temperatures. They are currently available in operations, but the need to drill deeper has significantly the standard T4 case size, measuring 26.97 mm long and boosted this range in recent years. As such, the components 10.31 mm in diameter with an insulating sleeve. This series employed in today’s wells must withstand extreme represents the largest standard case size for axial-leaded wet temperatures often exceeding 200°C at pressures greater tantalum capacitors and provides high CV ratings up to 330 than 25 kpsi. These components must also handle both µF/125 V. There are numerous emerging, short-lifetime applications continuous vibration up to 20 g and extreme shock spanning 100 –2,000 g. that demand high energy density after a period of latency The aerospace and defense markets also increasingly (e.g., a remote trigger). Such applications require that demand extreme-temperature passive components. For capacitors rest in a charged state for many months prior to instance, avionics continue to incorporate more sophisticated activation. These situations may be able to use parts rated electronics that continuously provide power, diagnostics, at 125°C, even if the environment periodically exceeds this and communications at temperatures exceeding those of temperature. In this regard, a new capacitor technology traditional automotive underhood applications. Unmanned targets low-frequency pulse applications. The capacitors aerial vehicles increasingly work in environments too have high CV ratings (up to 50 mF/ 6 V) that overlap with dangerous for manned flight. These critical systems demand supercapacitor technology but can operate continually at the utmost reliability where components must be capable of 125°C and exhibit extended lifetimes of up to 10,000 hours. withstanding temperatures exceeding the current military and aerospace standard of -55 to +125°C. TERMINATION MATERIAL OPTIONS Designers now have a range of reliable, highTantalum capacitors are available with a variety of temperature capacitor solutions to choose from, and can termination and metallization materials. High-temperature more easily and effectively match the unique performance capacitor designs incorporate metals with melting points that characteristics of their individual applications to the are well above the temperature rating of the capacitor, such appropriate capacitor as tin (Sn), palladium/silver (PdAg), and gold (Au). technology to achieve With regard to mounting, many solders used in better performance than commercial systems have low liquidus points that prevent ever before. their use at high temperatures. These applications require AVX Corp. high-melting-point (HMP) solder and/or high-temperature www.avx.com epoxies with a liquidus phase around 265°C. Above the liquidus phase temperatures, the solder enters a phase of
REFERENCES
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BATTERY TEST AND SIMULATION: WE’VE DONE THE HEAVY LIFTING
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Next Generation Battery Testing and Battery Simulation The 17040 is a highly efficient, precision battery test system. It’s regenerative, so power consumption is greatly reduced. The energy discharged from testing is recycled back to the grid reducing wasted energy and without generating harmonic pollution on other devices – even in dynamic charge and discharge conditions. It’s also dual mode. Not only is the 17040 equipped with a Charge/Discharge mode but has a Battery Simulation mode to verify if a connected device like a motor driver is functioning properly under varied conditions. Software/hardware integration and customization capabilities include BMS, data loggers, chambers, external signals, and HIL (Hardware in the Loop).
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